The bulky organoamides of alkali metals are used extensively as reagents in organic synthesis by virtue of the combination of their strong Bronsted basicity and their low nucleophilicity. Lithium dialkylamide and lithium alkylenecycloimide compounds such as lithium diisopropylamide (LDA), lithium hexamethyldisilazide (LHS) and lithium pyrrolidide (LPA), and are essentially insoluble in Lewis base-free hydrocarbon solvents. Although these compounds are soluble in ethers, they are quite unstable with time even at room temperature (except for LHS). Their stability can be improved by dissolving them in hydrocarbon solvents containing limited amounts of Lewis bases and further, by addition of small amounts of magnesium dialkylamides to these solutions.
The above described solutions may be prepared by reaction of the appropriate secondary amine in the desired solvent combination with either an alkyllithium, such as, e.g., n-butyllithium, or with lithium metal in the presence of an electron carrier, such as isoprene or styrene. In the latter case Lewis bases such as ethyl ether or tetrahydrofuran have been taught to be necessary for the metal reactions to proceed.
Recently, it was shown in U.S. Pat. No. 5,149,457 (EPO 423 087, 406,197) that certain secondary amines, such as, e.g., diisobutylamine, can be reacted with alkyllithium reagents such as, e.g., n-butyllithium, in purely hydrocarbon solvents to yield stable solutions of the corresponding lithium dialkylamide. However, these solutions contain the gaseous hydrocarbon by-products of the reaction, e.g., n-butane. These are not desirable because they increase the flammability of the product solutions and cause problems in subsequent solvent recycle. In addition, the process requires the use of the expensive organometallic reagent (viz., n-butyllithium).